BIG NO, NO !........listen, a 3-4 inch silver doesnt have nearly the same body mass as a 4-5 inch oscar. I would put your oscar in the 20 for the mean time till your arowana grows out....remember, silvers grow 1-2 inches a month, while the oscars are much shorter.....Just make sure to do pleanty of water changes for the oscar since its in such a small tank. Or if you already have a 150-200 gallon available toss the "O" in there. Let your silver get way bigger then your oscar before you put them together or but a tank seperater.